Flash memories and ferroelectric memories are known as non-volatile memories capable of retaining stored information even after power is turned off.
Among these, the flash memories have a floating gate embedded in a gate insulating film of an insulated-gate field-effect transistor (IGFET) to store information by accumulating electric charges indicating information to be stored in this floating gate. However, it is required for such a flash memory that a tunnel current is applied to the gate insulating film at the time of writing or erasing information. Thus, there is a disadvantage that a relatively high voltage is needed.
In contrast, the ferroelectric memories, which are also referred to as ferroelectric random access memories (FeRAMs), store information by utilizing hysteresis characteristics of a ferroelectric film with which a ferroelectric capacitor is provided. The ferroelectric film is polarized depending on a voltage applied between upper and lower electrodes of the capacitor, and spontaneous polarization remains even after the voltage is turned off. When polarity of an applying voltage is reversed, the spontaneous polarization is also reversed. Directions of spontaneous polarization are associated with “1” and “0”, so that information is written in the ferroelectric film. FeRAMs have advantages that a voltage required for this writing is lower than that required for the flash memories, and writing can be carried out at a higher speed than that of the flash memories.
The FeRAMs are broadly classified into stack-type FeRAMs and planar-type FeRAMs according to their structure. In the latter planar-type FeRAMs, a MOS transistor formed in a semiconductor substrate and a capacitor lower electrode are electrically connected through a metal wiring over a capacitor. Thus, it has a tendency that a planar shape of the capacitor tends to become larger.
In contrast, in the stack-type FeRAMs, a capacitor lower electrode is formed directly on a contact plug connected to a source/drain region of a MOS transistor, and the lower electrode and the MOS transistor are electrically connected through the contact plug. With such a structure, the planar shape of the capacitor can be made smaller than that of the planar-type FeRAMs. Thus, the stack-type FeRAMs have an advantage in miniaturization of FeRAMs which is expected to be required in the future.
As the contact plug of the stack-type FeRAMs, a tungsten plug is generally used. However, in patent literatures 1 to 4, there is disclosed a configuration that the contact plug is formed of a material other than tungsten.
For example, patent literature 1 discloses that the contact plug is formed of polycrystalline or amorphous silicon, and patent literature 2 discloses that the contact plug is formed of tungsten nitride. In addition, patent literature 3 discloses that the contact plug is formed of iridium, and patent literature 4 discloses that the contact plug is formed of iridium or ruthenium.
Note that a technology relating to the present embodiments is also disclosed in patent literature 5.
Patent literature 1: International Publication No. 97/33316, Pamphlet
Patent literature 2: Japanese Patent Application Laid-open Publication No. 2001-345432
Patent literature 3: Japanese Patent Application Laid-open Publication No. 2003-133534
Patent literature 4: Japanese Patent Application Laid-open Publication No. 2003-31775
Patent literature 5: Japanese Patent Application Laid-open Publication No. 2004-153031
Incidentally, when a general tungsten plug is employed as the above-described contact plug, a crystal orientation of tungsten affects an orientation of a lower electrode on the plug. Thereby, an orientation of a capacitor dielectric film is not oriented in a desired direction in some cases. If this is the case, ferroelectric characteristics of the capacitor dielectric film, such as residual polarization charges, are reduced. This is not preferable, since it is made difficult to perform the writing and reading operation in the capacitor.
In addition, when a tungsten plug is used as a contact plug as described above, there is a case where a conductive oxygen barrier film is formed between the contact plug and the lower electrode in order to prevent oxidation of tungsten. In this case, an orientation of the conductive oxygen barrier film is also affected by a crystal orientation of tungsten. Thus, there is a problem that the ferroelectric characteristics of the capacitor dielectric film are deteriorated, as in the case of the foregoing description.
Such a problem can be caused not only in the tungsten plug, but also in the contact plug in which crystalline material is used. Accordingly, the ferroelectric characteristics of the capacitor dielectric film are deteriorated even when the crystalline materials such as tungsten nitride, iridium, and ruthenium are used for the contact plugs as in the Patent literatures 2 to 4.
In addition, after the capacitor dielectric film is formed by patterning, annealing, which is referred to as recovery annealing, is carried out for the capacitor dielectric film in an oxygen atmosphere to compensate oxygen deficiency caused in the capacitor dielectric film at the time of the patterning. In the technology of patent literature 1 in which amorphous silicon is employed as the contact plug, contact resistance of the contact plug can be increased, because a surface of the contact plug is oxidized by the recovery annealing.